1. Field of the Invention
The present invention relates to electrodynamic type transducers that enable the transmission, within the sea, of acoustic waves and more particularly sound waves. These transducers are particularly useful in sonar technology.
2. Description of the Prior Art
It is the practice in underwater acoustics to use towed fish comprising electronic instruments and various transducers that can work in transmission, reception and possibly in both transmission and reception.
It is known that in order to be able to emit sufficient acoustic power at low frequencies, typically frequencies of 10 Hz to 1 kHz, it is necessary to move substantial masses of water. This requires a shift, itself substantial, of the active face of the transducer. This generally leads to the use, in this case, of an electrodynamic type transducer comprising a horn driven by a mobile coil located in a gap. Transducers of this type are thus quite similar to loudspeakers which are well known in musical acoustics.
To be able to obtain the acoustic power frequently needed in certain applications, given the acoustic level to be attained which can be as much as 150 dB at 10 Hz, it becomes necessary to use relatively large-sized transducers. This leads to constraints, in volume as well as in weight, because the transducer has to be immersed in the sea while being placed in a fish that has to navigate at a predetermined depth of immersion.
Moreover, the transducer often needs to be capable of withstanding the explosions that may sometimes occur in particular applications. The effect of an underwater explosion of this kind results in the application, to the transducer; of a level of hydrostatic pressure and acceleration. This level of hydrostatic pressure and acceleration is easily destructive at the horn and at the tight-sealing membrane between the horn and the transducer pack.
There is an electrical transducer for underwater acoustics more particularly known from the U.S. Pat. No. 4,466,083. This electrodynamic transducer can indeed be used to deliver high acoustic power, but is primarily designed to circumvent the problems due to heat dissipation corresponding to losses from electrical/acoustic conversion. The structure of this transducer does not enable it to resist underwater explosions. These explosions, if they occurred, would quickly make the transducer unusable by tearing its membrane, crushing its dome and causing deterioration to its draw-back springs.
In one known technique for resisting such explosions, a dome drilled with holes is placed over the horn of such a transducer, and the dome itself is covered with a membrane. Each of the holes thus forms a valve that lets through the vibrations corresponding to the acoustic signals emitted by the transducer, and does not let through the peaks of pressure that come from explosions if any. Such a system however has the disadvantage of increasing the volume and the mass of the transducer, and of decreasing the level of sound that it can deliver.